Caulobacter requires a dedicated mechanism to initiate chromosome segregation

Chromosome segregation in bacteria is rapid and directed, but the mechanisms responsible for this movement are still unclear. We show thatCaulobacter crescentus makes use of and requires a dedicated mechanism to initiate chromosome segregation.Caulobacter has a single circular chromosome whose origin of replication is positioned at one cell pole. Upon initiation of replication, an 8-kb region of the chromosome containing both the origin andparS moves rapidly to the opposite pole. This movement requires the highly conservedParABS locus that is essential inCaulobacter. We use chromosomal inversions andin vivo time-lapse imaging to show thatparS is theCaulobacter site of force exertion, independent of its position in the chromosome. WhenparS is moved farther from the origin, the cell waits forparS to be replicated before segregation can begin. Also, a mutation in the ATPase domain of ParA halts segregation without affecting replication initiation. Chromosome segregation inCaulobacter cannot occur unless a dedicatedparS guiding mechanism initiates movement.